The use of receivers in wireless systems such as radio and cellular communication systems is well-known in the art. FIG. 1 shows a typical quadrature receiver design 10. Receiver front-end processing 15 provides a signal such as an amplified intermediate frequency (IF) signal to mixers 20, 25. The front-end processing 15 can include, for example, an antenna that receives a radio frequency (RF) signal and provides the signal to an RF amplifier, which amplifies the RF signal. According to the example, the signal is then mixed in a mixer with a signal from a local oscillator to produce an IF signal that is amplified in an IF amplifier to produce the amplified IF signal. The IF signal is mixed with the quadrature outputs of a local oscillator 60 at mixers 20, 25 to generate respective in-phase (I) and quadrature (Q) signals. The I and Q signals are passed on their respective branches through, for example, low pass filters 30, 35 that eliminate the higher frequency components in the I and Q signals, and through one or more amplifiers 40, 45 to be digitized for further processing at respective analog-to-digital converters (ADCs) 50, 55. As is well known in the art, deviations from the ideal I and Q signals in the form of gain or magnitude and phase imbalances, i.e. differing gains for the I and Q signals as well as I and Q signals that are correlated due to the local oscillator 60 inputs at the mixers 20, 25 not being exactly 90 degrees out of phase, can cause degradations in the performance of the receiver.
Generally, prior attempts to calibrate and correct for magnitude and phase imbalances have involved analyzing dedicated transmitted calibration signals at the receiver to produce correction factors to be applied to received signals carrying data of interest, either at the analog side or the digital side. A representative example of the literature is R. A. Green, in “An Optimized Multi-tone Calibration Signal for Quadrature Receiver Communication Systems,” 10th IEEE Workshop on Statistical Signal and Array Processing, pp. 664–667, Pocono Manor, Pa., August 2000, which develops an optimized multi-tone calibration signal to which linear regression techniques are applied to generate correction factors to update adaptive filters that are intended to compensate for gain and phase imbalances. Special circuitry typically needs to be used to produce, analyze, and correct for the results of analysis on such calibration signals. A further drawback is that the quadrature receiver typically cannot continue to actively receive normal transmitted data while the calibration is occurring.
It would be desirable to provide I/Q calibration at a quadrature receiver that does not require that a separate calibration signal be transmitted to the receiver and that does not necessarily involve additional analog componentry prior to the ADCs.